This invention relates generally to ultrasonic flow rate measurement, and more particularly to an ultrasonic coupler assembly.
Ultrasonic flow meters are used to determine the flow rate (i.e., mean pipe flow rate (Vm)) of a variety of fluids (e.g., liquids, gases, etc.) in the fluid flowing in pipes of different sizes and shapes. In one type of ultrasonic flow meter employing transit time flow metering, one or more pairs of ultrasonic transducers can be attached to the exterior of the pipe wall, where each pair can contain ultrasonic transducers located upstream and downstream from each other, forming an ultrasonic path between them. Each ultrasonic transducer, when energized, transmits an ultrasonic signal (e.g., a sound wave) along an ultrasonic path through the flowing fluid that is received by and detected by the other ultrasonic transducer. The path velocity (i.e., path or chord velocity (Vp)) of the flowing fluid averaged along an ultrasonic path can be determined as a function of the differential between (i) the transit time of an ultrasonic signal traveling along the ultrasonic path from the downstream ultrasonic transducer upstream to the upstream ultrasonic transducer against the flow direction, and (2) the transit time of an ultrasonic signal traveling along the ultrasonic path from the upstream ultrasonic transducer downstream to the downstream ultrasonic transducer with the flow direction.
Knowledge of the flow rate of the fluid can enable other physical properties or qualities of the fluid to be determined. For example, in some custody-transfer applications, the flow rate can be used to determine the total volume (Q) of a fluid (e.g., water, oil, or gas) being transferred from a seller to a buyer through a pipe to determine the costs for the transaction, where the total volume is equal to the flow rate multiplied by the cross sectional area (A) of the pipe integrated over the time of flowing. In some applications (e.g., in refineries or nuclear power plants), the pipes to which the ultrasonic flow meters are attached are carrying high temperature fluids (e.g., coke in a refinery at 400° C.) that cause the pipe walls to also reach extremely high temperatures, or are carrying cryogenic fluids (e.g., liquefied natural gas) that cause the pipe walls to also reach extremely low temperatures. The ultrasonic transducers attached to those extreme temperature pipe walls are heated or cooled by the extreme temperatures of the pipes and therefore must be constructed of proper materials that increase the cost of these ultrasonic transducers. For example, an ultrasonic transducer rated at +200° C. or greater can be significantly more expensive than an ultrasonic transducer rated at +100° C. Similarly, an ultrasonic transducer rated at −200° C. or less can be significantly more expensive than an ultrasonic transducer rated at −100° C. In addition, even when constructed of the proper materials for extreme temperature applications, an ultrasonic transducer consistently exposed to extreme temperatures will experience thermal stresses that can diminish the useful life of the device.
In certain existing ultrasonic flow meter installations, an ultrasonic coupler is installed between the ultrasonic transducer and the pipe wall such that the ultrasonic transducer is not in direct contact with the pipe wall and therefore is not directly exposed to the extreme temperatures of the pipe wall. In this configuration, for example, one end of the ultrasonic coupler experiences the direct heat transfer from the pipe wall. The ultrasonic transducer attached at the other end of the ultrasonic coupler is not directly exposed to the extreme temperatures of the pipe wall, but instead is only exposed to the lower temperatures of the ultrasonic coupler closer to ambient room temperature. Therefore, the ultrasonic transducer can be rated to withstand a narrower temperature range. While these ultrasonic couplers can eliminate the need for an ultrasonic transducer rated at a higher temperature range, they can also introduce errors or uncertainty in the flow measurements provided by the ultrasonic flow meter. For example, certain ultrasonic couplers require that the ultrasonic signal beam transmitted by the ultrasonic transducer be redirected in the ultrasonic coupler between the ultrasonic transducer and pipe wall, introducing a possible error if the ultrasonic coupler is not manufactured according to strict tolerances and design criteria. Similarly, the ultrasonic signal traveling through certain ultrasonic couplers can be distorted or bent by isotherms formed in the ultrasonic coupler that change the direction of the ultrasonic signal beam. Therefore, there is a need to reduce the required temperature rating of an ultrasonic transducer without negatively impacting the accuracy of flow measurements provided by the ultrasonic flow meter.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.